1. Field of the Invention
The present invention generally relates to a scanning and imaging lens, an optical scanning device and an image forming apparatus.
2. Description of the Related Art
Optical scanning devices are widely used in xe2x80x98image forming apparatusxe2x80x99 such as a digital copier, an optical printer, an optical plate-making machine, a facsimile machine and so forth. A writing density of optical scanning devices has been increased to 1200 dpi, 1600 dpi, and is intended to be increased, further higher.
In order to achieve such high-density writing, it is necessary to form a beam spot having a small diameter, and, also, quality and stability of a beam spot is needed to be improved. Stability of a beam spot is determined from determining whether or not xe2x80x98a variation in beam-spot diameter on a surface to be scanned due to a variation in image heightxe2x80x99 is very small and stable. Quality of a beam spot is determined from determining whether or not xe2x80x98the light-intensity distribution of the beam spot has a simple mountain shape and does not have a complicated lower slope shapexe2x80x99.
In order to achieve a beam spot having high-quality and stability, it is necessary for a scanning and imaging optical system of an optical scanning device to have a high performance for forming a beam spot on a surface to be scanned using a deflected light flux. A factor causing a beam-spot diameter to fluctuate is, as is well known, xe2x80x98curvature of field in a scanning and imaging optical systemxe2x80x99, and many scanning and imaging optical systems in which curvature of field is well corrected have been proposed. Further, it is important for an optical magnification in a scanning and imaging optical system to be fixed when an image height of a beam spot changes.
However, in order to form a beam spot having stability and high quality, not only it is necessary to correct a geometrical optical performance such as curvature of field and an optical magnification but also it is important to xe2x80x98set a wave-optical wavefront aberration to be fixed between respective image heightsxe2x80x99.
An object of the present invention is to achieve high-density, satisfactory optical scanning with a stable and high-quality beam spot, by well correcting not only curvature of field and optical magnification but also xe2x80x98wavefront aberration on pupilxe2x80x99 in a scanning and imaging optical system.
A scanning and imaging lens according to the present invention is a xe2x80x98scanning and imaging lens which condenses a light flux deflected by a deflector onto a surface to be scanned as a beam spotxe2x80x99.
In the following description, for each of particular lenses constituting the scanning and imaging lens, a surface on the incidence side (surface on the side of deflector) is referred to as a first surface, and a surface on the exit side (surface on the side of surface to be scanned) is referred to as a second surface. However, when lens surfaces of the scanning and imaging lens are referred to in series from the side of deflector to the side of surface to be scanned in surface number, they will be expressed as xe2x80x98first surface, second surface, third surface, . . . xe2x80x99.
A scanning and imaging lens according to a first aspect of the present invention has the following features:
The scanning and imaging lens comprises xe2x80x98a plurality of lensesxe2x80x99.
These lenses have xe2x80x98shapes in main scanning cross section (imaginary plane cross section including an optical axis of a lens and in parallel to main scanning direction) such that a lens on the side of deflector (lens nearest to the deflector when the scanning and imaging lens comprises more than two lenses) has a convex shape in main scanning cross section in each of both surfaces, and has xe2x80x98at least one surfacexe2x80x99 having a non-arc shape in main scanning cross section, and a lens on the side of surface to be scanned (lens nearest to the surface to be scanned when the scanning and imaging lens comprises more than two lenses) has a convex shape in main scanning cross section in the first surface, and has xe2x80x98at least one surfacexe2x80x99 having a non-arc shape in main scanning cross section.
Further, the scanning and imaging lens has xe2x80x98at least two special toroidal surfacesxe2x80x99.
The xe2x80x98special toroidal surfacexe2x80x99 is a toroidal surface such that a curvature in sub-scanning cross section (imaginary plane cross section perpendicular to main scanning direction) varies in main scanning direction.
As will be described, there are lenses included in a scanning and imaging lens according to the present invention having a xe2x80x98surface shape not having a symmetry axisxe2x80x99. Therefore, an optical axis of a lens in the specification and claims denotes a reference axis in direction corresponding to an ordinary optical axis in an analytic expression determining the lens surface.
The scanning and imaging lens according to the first aspect of the present invention preferably has a lateral magnification xcex2o in sub-scanning direction on a light path from the origin of deflection by the deflector to the surface to be scanned when a chief ray passes through a point on the surface to be scanned at which an image height is 0 satisfying the following condition:
0.5 less than |xcex2o| less than 1.5xe2x80x83xe2x80x83(1)
Further, the scanning and imaging lens preferably has the above-mentioned lateral magnification xcex2o and a lateral magnification xcex2h in sub-scanning direction when a chief ray passes through the surface to be scanned at which an image height is an arbitrary amount satisfying the following conditon:
0.9 less than |xcex2h/xcex2o| less than 1.1xe2x80x83xe2x80x83(2)
Further, xe2x80x98at least one surface of the special toroidal surfacesxe2x80x99 in the scanning and imaging lens may have a non-arc shape in main scanning cross section, and, also, a non-arc shape in sub-scanning cross section, wherein the non-arc shape in sub-scanning cross section may vary according to a coordinate Y in main scanning direction, and the non-arc shape in sub-scanning cross section in each coordinate Y may be determined so as to correct wavefront aberration on the surface to be scanned, and wherein at least one surface of the special toroidal surfaces may be such that xe2x80x98a curvature in sub-scanning cross section varies asymmetrically with respect to the optical axis in main scanning directionxe2x80x99.
Further, the scanning and imaging lens may comprise two single lenses L1 and L2 disposed from the side of deflector to the side of surface to be scanned.
Each of the lenses L1 and L2 of the scanning and imaging lens may have a xe2x80x98meniscus shape such that the side of deflector is concave and the side of surface to be scanned is convexxe2x80x99 in sub-scanning cross section on the optical axis and in the proximity thereof.
Each of the lenses L1 and L2 preferably has a positive power in main scanning cross section, and a radius Rm2 of curvature of the second surface of the lens L1 on the optical axis in main scanning cross section and a radius Rm3 of curvature of the first surface of the lens L2 on the optical axis in main scanning cross section preferably satisfy the following condition:
0.1 less than |Rm2/Rm3| less than 0.5xe2x80x83xe2x80x83(3)
Further, a radius Rs2 of curvature of the second surface of the lens L1 on the optical axis in sub-scanning cross section and a radius Rs4 of curvature of the second surface of the lens L2 on the optical axis in sub-scanning cross section preferably satisfy the following condition:
0.05 less than |Rs4/Rs2| less than 0.5xe2x80x83xe2x80x83(4)
The scanning and imaging lens may have at least two special toroidal surfaces in each of which xe2x80x98a curvature in sub-scanning cross section varies asymmetrically with respect to the optical axis in main scanning directionxe2x80x99, and each of the lenses L1 and L2 may have xe2x80x98at least one of these special toroidal surfacesxe2x80x99.
An optical scanning device according to the present invention is an xe2x80x98optical scanning device deflecting a light flux from a light source by a deflector, condensing the deflected light flux onto a surface to be scanned as a beam spot by a scanning and imaging lens, and, thus, scanning the surface to be canned by the beam spotxe2x80x99.
Any of the above-described scanning and imaging lenses according to the first aspect of the present invention may be used as the scanning and imaging lens of the optical scanning device according to the present invention.
As the above-mentioned deflector, xe2x80x98one having a deflection reflective surfacexe2x80x99 such as rotational polygon mirror, rotational bi-surface mirror, rotational mono-surface mirror, or the like may be used. In this case, the light flux from the light source forms on or in the proximity of the deflection reflective surface a xe2x80x98line image long in main scanning directionxe2x80x99 by a xe2x80x98line-image forming optical systemxe2x80x99 such as a cylindrical lens, cylindrical concave mirror, or the like. Thereby, it is possible to correct xe2x80x98inclination of surfacexe2x80x99 of the deflector.
As the light source, a semiconductor laser may be preferably used. A light flux emitted by the light source may be transformed into a xe2x80x98substantially parallel light fluxxe2x80x99 by a coupling lens. It is possible to transform the light flux obtained from coupling performed by the coupling lens into a xe2x80x98divergent light flux or convergent light fluxxe2x80x99.
In the optical scanning device, a spot diameter according to LSF (Line Spread Function) of 1/e2 intensity of a beam spot condensed onto the surface to be scanned may be set to equal to or smaller than 50 xcexcm in each of main and sub-scanning directions.
That is, in a position of image height of the beam spot on the surface to be scanned, a coordinate xcex7 is set in main scanning direction and a coordinate xcex6 is set in sub-scanning direction, an intensity distribution F(xcex6, xcex7) of the beam spot approximated as a two-dimensional Gaussian distribution is integrated so that P(xcex6)=∫F(xcex6, xcex7)dxcex7 is obtained. Then, xe2x80x98the width of the range of xcex6xe2x80x99 through which P(xcex6) is equal to or larger than 1/e2 after the maximum of P(xcex6) is normalized into 1 is regarded as the spot diameter in sub-scanning direction. With regard to main scanning direction, the same manner is applied.
An image forming apparatus according to the present invention is an xe2x80x98image forming apparatus forming an electrostatic latent image on a latent-image carrying body through optical scanning, visualizing the formed electrostatic latent image, and obtaining a desired recorded imagexe2x80x99.
Any of the above-described optical scanning devices may be used as an optical scanning device performing the optical scanning of the latent-image carrying body of the image forming apparatus according to the present invention.
In this case, a photoconductive photosensitive body may be used as the latent-image carrying body, the electrostatic latent image may be formed thereon through uniform charging and the optical scanning thereof, and the thus-formed electrostatic latent image may be visualized as a toner image. The toner image is fixed onto a sheet-like recording medium (transfer paper, plastic sheet for an overhead projector, or the like).
In the image forming apparatus, a film for photography with silver halide may be used as the image carrying body, for example. In this case, the electrostatic latent image formed through the optical scanning by the optical scanning device is visualized by a xe2x80x98method of developing in an ordinary process of photography with silver halidexe2x80x99. Such an image forming apparatus may be embodied as an xe2x80x98optical plate-making systemxe2x80x99, a xe2x80x98medical image forming apparatusxe2x80x99 in which a result of CT scan or the like is output to a film for photography with silver halide, or the like.
The image forming apparatus according to the present invention may also be embodied as a laser printer, a laser plotter, a digital copier, a facsimile machine, or the like.
For obtaining a small-diameter, satisfactory and stable beam spot, a scanning and imaging lens needs to satisfy the following conditions:
{circle around (1+L )} Curvature of field in main scanning direction and sub-scanning direction is well corrected;
{circle around (2+L )} wavefront aberration is well corrected; and
{circle around (3+L )} Optical magnification is fixed through respective image heights.
As mentioned above, the scanning and imaging lens according to the first aspect of the present invention comprises xe2x80x98a plurality of lensesxe2x80x99. These lenses have xe2x80x98shapes in main scanning cross section such that a lens on the side of deflector has a convex shape in main scanning cross section in each of both surfaces, and has xe2x80x98at least one surfacexe2x80x99 having a non-arc shape in main scanning cross section, and a lens on the side of surface to be scanned has a convex shape in main scanning cross section in the first surface, and has xe2x80x98at least one surfacexe2x80x99 having a non-arc shape in main scanning cross section. Further, the scanning and imaging lens has xe2x80x98at least two special toroidal surfacesxe2x80x99.
Because thus the first surface (exit surface) of the lens on the side of surface to be scannedxe2x80x99 of the scanning and imaging lens is convex on the side of deflector in main scanning cross section, it can be prevented in the lens on the side of surface to be scanned that xe2x80x98a radius of curvature in sub-scanning cross section in the lens and in proximity thereof is reduced too muchxe2x80x99. Thereby, it is possible to make the optical magnification be fixed for respective image heights.
When a radius of curvature in sub-scanning cross section is reduced too much, wavefront aberration is likely to be generated in particular in peripheral image heights.
When the above-described lens shape is employed, a sign of coma aberration generated in the lens on the side of deflector (in particular, in the first surface thereof) is reversed to a sign of coma aberration generated in the lens on the side of surface to be scanned (because the inclination of the coma aberration is reverse). Accordingly, the coma aberrations are cancelled out one another, and, thereby, wavefront aberration can be well corrected.
Further, because each of the lenses on the side of deflector and on the side of surface to be scanned has at least one surface having a non-arc shape in main scanning cross section, it is possible to well correct curvature of field in main scanning direction. In each of first and second embodiments described later, the shape of the surface (second surface of the lens nearest to the surface to be scanned) nearest to the surface to be scanned in main scanning cross section is a concave shape. However, the shape of this surface in main scanning cross section may be a convex shape or may be a plane shape. That is, the xe2x80x98shape in main scanning cross sectionxe2x80x99 of the lens on the side of surface to be scanned may be of both-convex, convex-plane or meniscus shape.
Further, although the light flux emitted from the coupling lens is a xe2x80x98parallel light fluxxe2x80x99 in each of the first and second embodiments, similar performance can be obtained when it is either a divergent light flux or a convergent light flux.
When the upper limit of the condition (1) is exceeded, the imaging magnification of the scanning and imaging lens increases. Thereby, when the spot diameter on the image surface is attempted to be reduced, the exit pupil in sub-scanning direction will be made too much larger in diameter. Thereby, it will be difficult to correct the wavefront aberration on the pupil through the entirety thereof. Further, a necessity of improving the NA of the coupling lens will occur. Further, change in position of image surface occurring due to change in environment and/or error in accuracy of mounting lenses will increase, and, thereby, it will be difficult to reduce the spot diameter.
When the lower limit of the condition (1) is exceeded, the magnification decreases too much, and, thereby, it is advantageous for correcting the wavefront aberration in comparison to a case where the magnification increases, because the aperture diameter becomes smaller. However, the efficiency in transmission of light toward the surface to be scanned becomes lowered, and, thereby, high-speed writing becomes difficult.
When the above-described scanning and imaging lens according to the first aspect of the present invention is used, the xe2x80x98beam waist diameterxe2x80x99 to be the spot diameter changes approximately in proportion to a change in the lateral magnification of the scanning and imaging lens. Therefore, in order to obtain a small-diameter, stable beam spot, it is essential to make the lateral magnification of the scanning and imaging lens for respective image heights be uniform. The condition (2) is a condition determining xe2x80x98allowance in changexe2x80x99 of the above-mentioned lateral magnification.
The scanning and imaging lens according to the present invention includes two xe2x80x98special toroidal surfacesxe2x80x99. In the special toroidal surface, a curvature in sub-scanning cross section varies in main scanning direction. Accordingly, it is possible to freely change, in main scanning direction, the positions of principal points on the front and rear sides in sub-scanning direction. Thereby, it is possible to maintain a uniform magnification for respective image heights, and, to achieve a stable beam spot. A shape in sub-scanning cross section of the above-described special toroidal surface may be either an arc shape or a non-arc shape.
In each of the first and second embodiments described later, a first lens (lens on the side of deflector) and a second lens (lens on the side of surface to be scanned) have a uniform magnification through setting of positions of principal points through bending.
The above-mentioned condition (2) is an advantageous condition also in a xe2x80x98multi-beam scanning devicexe2x80x99, and, as a result of making magnification for respective image heights uniform, it is possible to maintain a scanning line pitch interval satisfactorily (for example, 21.2 xcexcm in a case of 1200 dpi, adjacent scanning).
In the scanning and imaging lens, as described above, xe2x80x98at least one surface of the special toroidal surfacesxe2x80x99 may have a non-arc shape in main scanning cross section, and, also, a non-arc shape in sub-scanning cross section, wherein the non-arc shape in sub-scanning cross section may vary according to a coordinate Y in main scanning direction, and the non-arc shape in sub-scanning cross section in each coordinate Y may be determined xe2x80x98so as to correct wavefront aberration on the surface to be scannedxe2x80x99. Thereby, it is possible to well correct the curvature of field in each of main scanning direction and sub-scanning direction for each image height of scanning, also, to positively correct the wavefront aberration on the pupil for each image height, and, thereby, to obtain a small-diameter, stable, high-quality beam spot.
In each of the first, second and third embodiments which will be described later, the second surface (exit surface) of the lens on the side of surface to be scanned is a xe2x80x98special toroidal surface in which the shape in sub-scanning cross section is a non-arc shape, and, also, the non-arc shape varies asymmetrically in main scanning directionxe2x80x99. However, this toroidal surface may also be employed as another surface of the scanning and imaging lens.
As the deflector, a rotational polygon mirror is common. However, in the rotational polygon mirror, xe2x80x98the center of rotation of a deflection reflective surface is not located in the deflection reflective surfacexe2x80x99. Thereby, the origin of deflection of a deflected light flux by the deflection reflective surface changes as the deflection reflective surface rotates. As a result, optical sag is generated, which results in degradation in curvature of field in sub-scanning direction in particular.
By making the curvature in sub-scanning cross section of the special toroidal surface vary xe2x80x98asymmetrical with respect to the optical axisxe2x80x99 in main scanning direction as mentioned above, it is possible to effectively correct the above-mentioned degradation in curvature of field due to sag, and, thereby, to obtain a stable beam spot.
Conventionally, when a beam spot having a small diameter equal to or shorter than 50 xcexcm is attempted to be obtained, more than two single lenses are needed in order to achieve necessary optical characteristics because the optical characteristics such as wavefront aberration, curvature of field, uniform-velocity characteristics (fxcex8 characteristics), optical magnification and so forth should be well improved. However, according to the present invention, a small-diameter, high-stability beam spot can be obtained by a configuration of two single lenses.
By configuring each of the lenses L1 and L2 of the scanning and imaging lens to cause it to have a xe2x80x98meniscus shape such that the side of deflector is concave, the side of surface to be scanned is convex and a positive power is obtained in sub-scanning cross section on the optical axis and in the proximity thereofxe2x80x99, it becomes easy to locate the principal point forward, and it becomes easy to have a configuration such that the magnification is uniform.
Further, as a result of configuring each of the lenses L1 and L2 to cause it to have a positive power, it is possible to set a xe2x80x98radius of curvature in sub-scanning cross section of each surfacexe2x80x99 to a large value, and, thereby, to well correct the wavefront aberration while maintaining a uniform magnification.
When the upper limit of the above-mentioned condition (3) is exceeded, a central thickness of the lens L2 is large, and, also, it is difficult to form an edge of the lens properly through plastic molding. When the lower limit thereof is exceeded, it is difficult to well correct the uniform-velocity characteristics (fxcex8 characteristics and linearity) and curvature of field in main scanning direction.
When the upper limit of the above-mentioned condition (4) is exceeded, degradation in wavefront aberration occurs, and it is difficult to obtain a small-diameter, satisfactory beam spot. When the lower limit thereof is exceeded, it is difficult to maintain a uniform magnification.
As described above, in the scanning and imaging lens according to the first aspect of the present invention, it is possible to well correct influence of sag, and, by employing a plurality of surfaces (asymmetrical surfaces) in each of which xe2x80x98a paraxial curvature on sub-scanning cross section is set to vary asymmetrical along main scanning directionxe2x80x99, it is possible to well correct both the curvature of field and optical magnification in sub-scanning direction. In comparison to employing the above-mentioned xe2x80x98asymmetrical surfacesxe2x80x99 in the same lens, a larger advantage can be obtained when they are set in surfaces which are apart from each other.
A scanning and imaging lens according to a second aspect of the preset invention has the following features:
This scanning and imaging lens consists of a plurality of lenses.
The lens on the side of deflector (lens nearest to the deflector when the scanning and imaging lens consists of more than two lenses) has xe2x80x98a positive power and at least one surface having a non-arc shapexe2x80x99 in main scanning cross section.
The lens on the side of surface to be scanned (lens nearest to the surface to be scanned when the scanning and imaging lens consists of more than two lenses) has xe2x80x98the first surface having a convex shape and at least one surface having a non-arc shapexe2x80x99 in main scanning cross section.
Further, this scanning and imaging lens has at least two xe2x80x98special toroidal surfaces in each of which a curvature in sub-scanning cross section varies in main scanning directionxe2x80x99.
Different from the scanning and imaging lens according to the first aspect of the present innovation in which the lens on the side of deflector has xe2x80x98both surfaces having convex shapes in main scanning cross sectionxe2x80x99, the lens on the side of deflector of this scanning and imaging lens according to the second aspect of the present invention may have xe2x80x98a plane-convex shape or a meniscus shapexe2x80x99 other than a xe2x80x98both convex shapexe2x80x99 in main scanning cross section because the conditon thereof is such as to have a xe2x80x98positive power in main scanning cross sectionxe2x80x99. Accordingly, the scanning and imaging lens according to the second aspect of the present invention implies the scanning and imaging lens according to the first aspect of the present invention.
As later described in description of the third embodiment, in the scanning and imaging lens according to the second aspect of the present invention, xe2x80x98both the lens on the side of deflector and lens on the side of surface to be scanned may have meniscus shapes in main scanning cross section, and be disposed so that the convex surfaces thereof are contiguous to one another in main scanning cross sectionxe2x80x99.
Further, also this scanning and imaging lens preferably has the above-described xe2x80x98lateral magnification xcex2o in sub-scanning direction on a light path from the origin of deflection by the deflector to the surface to be scanned when a chief ray passes through the surface to be scanned at which an image height is 0xe2x80x2 satisfying the following condition:
0.5 less than |xcex2o| less than 1.5xe2x80x83xe2x80x83(1)
Further, the scanning and imaging lens preferably has the above-mentioned lateral magnification xcex2o and a lateral magnification xcex2h in sub-scanning direction when a chief ray passes through the surface to be scanned at which an image height is an arbitrary amount satisfying the following conditon:
0.9 less than |xcex2h/xcex2o| less than 1.1xe2x80x83xe2x80x83(2)
The significance of satisfaction of these conditions (1) and (2), and the fact that these conditions are advantageous conditions also in a multi-beam scanning device are same as in the case of the first aspect of the present invention.
Further, same as in the case of the first aspect of the present invention, xe2x80x98at least one surface of the special toroidal surfacesxe2x80x99 in the scanning and imaging lens in the second aspect of the present invention may have a non-arc shape in main scanning cross section, and, also, a non-arc shape in sub-scanning cross section, wherein the non-arc shape in sub-scanning cross section may vary according to a coordinate Y in main scanning direction, and the non-arc shape in sub-scanning cross section in each coordinate Y may be determined so as to correct wavefront aberration on the surface to be scannedxe2x80x99.
Further, any of the scanning and imaging lenses according to the second aspect of the present invention may comprise two single lenses L1 and L2 disposed from the side of deflector to the side of surface to be scanned. In this case, when X3 (D3) denotes a space (interval) between the lens L1 on the side of deflector and lens L2 on the side of surface to be scanned, and X4 (D4) denotes a thickness of the lens L2 on the side of surface to be scanned, these preferably satisfy the following condition:
0.10 less than X4/X3 less than 0.30xe2x80x83xe2x80x83(5)
When the upper limit 0.30 of the conditon (5) is exceeded, a thickness of the lens L2 on the side of surface to be scanned is likely to be large, xe2x80x98distortion is likely to be generated in a surface and/or inside of the lens at a time of moldingxe2x80x99 when the lens is made of plastic, and, thereby, the wavefront aberration may be degraded. When the lower limit 0.10 is exceeded, a thickness of the lens L2 is too small, and, it is difficult to well correct the curvature of field and fxcex8 characteristics in main scanning direction.
Further, by the reason same as in the case of the scanning and imaging lens according to the first aspect of the present invention, also in the scanning and imaging lens according to the second aspect of the present invention, each of the lenses L1 and L2 of the scanning and imaging lens may preferably have a xe2x80x98meniscus shape such that the side of deflector is concave and the side of surface to be scanned is convexxe2x80x99 in sub-scanning cross section on the optical axis and in the proximity thereof.
Also another optical scanning device according to the present invention is an xe2x80x98optical scanning device deflecting a light flux from a light source by a deflector, condensing the deflected light flux onto a surface to be scanned as a beam spot by a scanning and imaging lens, and, thus, scanning the surface to be scanned by the beam spotxe2x80x99. Any of the above-described scanning and imaging lens according to the second aspect of the present invention may be used as the scanning and imaging lens of this optical scanning device.
As the above-mentioned deflector, same as in the case described above, xe2x80x98one having a deflection reflective surfacexe2x80x99 such as rotational polygon mirror, rotational bi-surface mirror, rotational mono-surface mirror, or the like may be used. In this case, the light flux from the light source forms on or in the proximity of the deflection reflective surface a xe2x80x98line image long in main scanning directionxe2x80x99 by a xe2x80x98line-image forming optical systemxe2x80x99 such as a cylindrical lens, cylindrical concave mirror, or the like. Thereby, it is possible to correct xe2x80x98inclination of surfacexe2x80x99 of the deflector.
As the light source, same as in the case described above, a semiconductor laser may be preferably used. A light flux emitted by the light source may be transformed into a xe2x80x98substantially parallel light fluxxe2x80x99 by a coupling lens. It is possible to transform the light flux obtained from coupling performed by the coupling lens into a xe2x80x98divergent light flux or convergent light fluxxe2x80x99.
In the optical scanning device, same as in the case described above, a spot diameter according to LSF (Line Spread Function) of 1/e2 intensity of a beam spot condensed onto the surface to be scanned may be set to equal to or smaller than 50 xcexcm in each of main scanning direction and sub-scanning direction. Thereby, high-density optical scanning by the optical scanning device is made possible.
In the case where any of the scanning and imaging lenses having the two lenses L1 and L2 according to the second aspect of the present invention is used in any of these optical scanning devices, X3 and L preferably satisfy the following condition:
0.15 less than X3/L less than 0.30xe2x80x83xe2x80x83(6)
when X3 (D3) denotes the space (interval) between the lens L1 on the side of deflector and lens L2 on the side of surface to be scanned, and L denotes a distance between the origin of deflection by the deflector and the surface to be scanned.
The condition (6) is a condition for improving the curvature of field and fxcex8 characteristics in main scanning direction of the scanning and imaging lens in the optical scanning device. When the upper limit 0.30 is exceeded, it is difficult to well correct the curvature of field and fxcex8 characteristics in main scanning direction. When the lower limit 0.15 is exceeded, the lateral magnification in sub-scanning direction is likely to increase, and it is difficult to well correct the wavefront aberration.
Another image forming apparatus according to the present invention is also an xe2x80x98image forming apparatus forming an electrostatic latent image on a latent-image carrying body through optical scanning, visualizing the formed electrostatic latent image, and obtaining a desired recorded imagexe2x80x99. Any of the above-described optical scanning devices may be used as an optical scanning device performing the optical scanning of the latent-image carrying body of this image forming apparatus.
Also in this case, same as in the case described above, a photoconductive photosensitive body may be used as the latent-image carrying body, the electrostatic latent image may be formed thereon through uniform charging and the optical scanning thereof, and the thus-formed electrostatic latent image may be visualized as a toner image. The toner image is fixed onto a sheet-like recording medium (transfer paper, plastic sheet for an overhead projector, or the like).
In the image forming apparatus, a film for photography with silver halide may be used as the image carrying body, for example. In this case, the electrostatic latent image formed through the optical scanning by the optical scanning device is visualized by a xe2x80x98method of developing in an ordinary process of photography with silver halidexe2x80x99. Such an image forming apparatus may be embodied as an xe2x80x98optical plate-making systemxe2x80x99, a xe2x80x98medical image forming apparatusxe2x80x99 in which a result of CT scan or the like is output to a film for photography with silver halide, or the like.
This image forming apparatus according to the present invention may also be embodied as a laser printer, a laser plotter, a digital copier, a facsimile machine, or the like.
Other objects and further features of the present invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings.